In a magnetic disk device, size of a magnetic particle composing a recording layer of a disk has been micronized. In a magnetic disk device, as a magnetic particle is micronized, thermal stability decreases, and thermal fluctuation in which a magnetization direction of the magnetic particle freely inverts by thermal energy, may occur. When the magnetization direction of the magnetic particle has inverted due to the occurrence of the thermal fluctuation, the magnetic disk device may not correctly read data.